Conventional taper thread deformed reinforcing bar couplers have been sold for many years throughout the world under the trademark LENTON®. LENTON® is a registered trademark of ERICO INTERNATIONAL Corporation of Solon, Ohio, U.S.A. Taper threads are preferred because of the ease of assembly requiring only a few turns of the sleeve coupler or bar and the ability to avoid cross threading and subsequent damage to the threads.
The threading process cuts the taper threads in the deformed bar end including the nominal diameter and any projecting ribs or deformations. The process however notches the bar and such couplings will not normally achieve bar break tensile capability.
In order to achieve higher tensile strength bar splices it has been attempted literally to upset the bar end to obtain a larger diameter end section which then receives a tapered or straight thread which has a larger pitch diameter than the nominal diameter of the bar. In the case of tapered threads the average thread diameter is larger than the bar nominal diameter. Such bars can achieve bar break but at a considerable cost in energy and handling. To achieve such upset bar end, the bar end literally has to be forged with substantial axial force or forge hammering. This is complicated by the fact that reinforcing bar, when cut, generally has a bent end caused by shear equipment, and if the bars are of any length or size the handling and conveying problems result in very high cost bar splices to achieve the desired minimal increase in strength.
A published U.K. Patent Application No. 2 227 802A illustrates a tapered thread bar splice having an enlarged or upset tapered threaded end. More importantly this published patent illustrates the sizable machinery including a large ram and clamps required to upset the bar end all prior to threading. The operation is simply not something that can be done easily, locally, or at a construction or fabrication site. Also to be economical the operation requires large volumes of inventory and careful handling and transportation.
Another simplified example of the type of machinery required is seen in U.S. Pat. No. 5,660,594.
Examples of such prior devices involving high cost forging or upsetting are seen in LENTON® continuity sets sold by applicant. The splices involve tapered threads on forged or upset bar ends.
Straight thread couplers on forged or upset bar ends are seen in U.S. Pat. Nos. 4,619,096, 5,158,527, and 5,152,118.
CCL Systems of Leeds, England also markets a BARTEC system where the bar ends have been enlarged and threaded to mate with parallel sleeve threads.
A coupling similar to that of the above U.K. published patent application is shown in Chinese published application 97107856.4.
It has however been discovered that similar tensile benefits can be achieved without the necessity of the costly upsetting or enlargement of the bar end.